The present invention relates to a switching circuit of a switching type power conversion apparatus. In particular, the present invention relates to a type of switching circuit in which a driving transistor is connected to a switching main transistor to supply a driving power for ON-OFF driving thereto in response to a control signal, and an auxiliary power source is provided between the main transistor and the driving transistor.
In view of efficient utilization of energy, a power conversion apparatus using a semiconductor switching element or switching transistor has an extremely widespread availability due to its excellent characteristics in power conversion efficiency. The semiconductor switching transistor includes a voltage-driven type transistor, such as an isolated-gate bipolar transistor (IGBT), static-induction transistor (SIT) and field-effect transistor (FET), and a current-driven type transistor, such as a bipolar-mode static-induction transistor (BSIT) and bipolar junction transistor (BJT).
The voltage driven type transistor may be directly driven by a voltage signal so that a driving circuit may be readily simplified and its driving frequency may also be arranged higher. In applications requiring a withstand voltage of 250V or more, several types of switching transistors are selectively used depending on requirements for capacity and driving frequency. Specifically, in case of using the switching transistors in a driving frequency range of several KHz to several hundred KHz, the IGBT excellent in overall balance of voltage drop in ON state and switching performance and the FEA having small current capacity but capable of high speed operation are widely employed in the power conversion apparatus.
On the other hand, since the current-driven switching type transistor is driven by applying current to a control terminal, a driving circuit tends to be complexified and to have a lower operation speed than that of the voltage-driven type transistor. However, the current-driven type switching transistor has an advantageous feature that the voltage drop in ON state is about one-third to one-sixth of that of the voltage-driven type transistor, and thereby provides a lower conduction loss. This proves that the current-driven type switching transistor is more suitable for providing a downsized power conversion apparatus.
While there are broadly classified two types of semiconductor switching elements or switching transistor available for the power conversion apparatus, as described above, it has been often the case that the voltage-driven type switching transistor having a low switching loss and facilitating a high frequency driving was employed in view of downsizing of components, simplification of circuits, downsizing based on high driving frequency, cost reduction and other. However, considering how to coping with social needs for achieving an enhanced efficiency and downsizing with an eye to the future, the level of voltage drop in ON state of the voltage-driven type element will be an obstacle as long as holding over the technique using the current voltage-driven switching transistor. In particular, observing the current situation, the voltage drop in ON state of the IGBT et al. being a mainstream voltage-driven switching transistor has already been improved closely up to the theoretical value. All the more because of its current high percentage of completion, it cannot be expected to reduce the conduction loss drastically.
As to switching loss, loss recovery techniques utilizing resonance phenomenon and soft switching techniques have been developed for preventing electromagnetic environment pollution and reducing power loss. In contrast, a conduction loss in transistors serving as a switching element inevitably arises when a current is passed through the element and the level of the loss depends on the performance of the element. Thus, the conduction loss cannot be readily reduced only by a simple modification but a radical review of circuit topology.
In the technical field of the power conversion apparatus, various efforts are currently continued to achieve further downsized apparatus as a whole, higher power density, and higher efficiency et al.
Two primary losses arise in the semiconductor switching transistor of the power conversion apparatus; one is a switching loss arising in the course of changing the transistor from ON state to OFF state or from OFF state to ON state; and the other is a conduction loss caused by a voltage drop arising in the transistor when this transistor is in ON state. Thus, in order to provide a power conversion apparatus capable of meeting the need in response to the demand for further downsizing the current power conversion apparatus and enhancing its power density, it is necessary to develop a technique capable of achieving higher efficiency by comprehensively reducing both of the conduction loss caused by the voltage drop in ON state of the switching transistor and the switching loss which lead to a power loss.
Heretofore, there have been very few cases reporting that the conduction loss in the switching transistor was reduced by an effective improvement in circuit. Giving some examples from among such few cases, Japanese Patent Laid-Open Publication No. Hei 1-97173 discloses a technology for reducing both a switching loss and conduction loss in a PWM full-bridge power conversion apparatus, such as a PWM inverter, by applying a semiconductor switching element having a small conduction loss, such as a bipolar transistor, to an arm switched by commercial frequency, and a semiconductor switching element having a small switching loss, such as a static-induction transistor (SIT), to an arm switched by high-frequency, so as to make up a bridge circuit in the apparatus. The Journal of the Institute of Electrical Engineers of Japan, Section D, vol. 116, No. 12, 1996, pp. 1205-1210, also discloses a modification in circuit for reducing a conduction loss in a power conversion apparatus using semiconductor switching elements. However, these prior arts involve problems, such as an actual restriction of their driving frequency, due to insufficient studies in terms of optimization of the conduction loss, reduction of the loss in their driving circuit, downsizing et al. For example, the aforementioned Japanese Patent Laid-Open Publication includes no specific teaching about how to drive the bipolar transistor serving as a current control switching element. However, when a constant current is applied to a base of the transistor as in conventional methods for driving transistors, the efficiency in low load will be particularly deteriorated due to the driving loss in no load state or low load state. In the technique described in the aforementioned Journal of the Institute of electrical Engineers of Japan, two transistors each having a small conduction loss are selectively used among the switching transistors to couple with each other in the form of the Darlington-connection, and its initial-stage transistor serves as a driving transistor. Further, an auxiliary power source composed of a current transformer (CT) is interposed between the driving transistor and the other or main transistor. This disclosure describes that this circuitry may reduce the conduction loss to one-third. However, in the circuit described in this disclosure, it is necessary for the couple of Darlington-connected transistors to have a high withstand voltage characteristic. Generally, as a withstand voltage of a semiconductor switching element is increased, the element has an increased voltage drop and a lowered switching speed. Thus, this technique has its limits in achieving an improved efficiency and enhanced driving frequency.
In view of the aforementioned problems in the prior arts, it is an object of the present invention to provide a switching circuit for a power conversion apparatus capable of reducing conduction loss to provide a higher efficiency, and achieving downsizing and weight-reduction and higher driving frequency based on the improved efficiency.
In order to achieve the aforementioned object, in a switching circuit for a power conversion apparatus according to the present invention, a driving transistor is connected to a switching main transistor to supply a driving power for ON-OFF driving thereto, and an auxiliary power source composed of a current transformer is provided between the main transistor and the driving transistor. Further, an auxiliary transistor having a lower switching loss than that of the main transistor is connected in parallel with the main transistor to form a main switch in combination with the main transistor. In the present invention, a current-driven type transistor serves as the main transistor, and voltage-driven type transistors serve as both of the driving transistor and the auxiliary transistor. The auxiliary transistor is adapted to be driven at a higher speed timing than that of the main transistor when the main transistor is turned on, and adapted to be driven at a lower speed than that of the main transistor when the main transistor is turned off.
In a preferred embodiment of the present invention, the main transistor is adapted to have a period of OFF state in the state when a driving control signal is transmitted only to the auxiliary transistor to bring the auxiliary transistor into ON state. In this case, a regenerative diode is preferably provided to regenerate power from an output of the main switch to the auxiliary power source in the period when the driving control signal is transmitted only to the auxiliary transistor. In another embodiment of the present invention, the auxiliary transistor may be adapted to be driven only during an activation period of the main switch. Further, in another embodiment of the present invention, an activating power source may be provided for supplying an activating power to the auxiliary power source only during the activation period of the main switch.
In another aspect of the present invention, there is provided a switching circuit for a power conversion apparatus, wherein a driving transistor is connected to a switching main transistor to supply a driving power for ON-OFF driving thereto, and an auxiliary power source composed of a current transformer is provided between the main transistor and the driving transistor, so as to supply a power from the auxiliary power source to the driving transistor through a rectifier circuit. In this case, a current-driven type transistor serves as the main transistor, and voltage-driven type transistors serves as the driving transistor. An activating device is also provided for applying a bias power to the auxiliary power source in an earlier timing than that of an activation of the main transistor when the main transistor is turn on.